The invention relates to a method and an arrangement for a vibration-resistant playback device having a reduced number of returns and having a reduced power consumption. The field of application of vibration-resistant playback devices applies, in particular, to the portable and mobile use of playback devices of optical information media, in which uninterrupted and undisturbed reproduction of information must be ensured despite vibrations, the field of application not being restricted, however, to optical information media.
Optical scanning devices, in particular, are sensitive to mechanical vibrations, since the optical scanning system can be pitched from the track by vibrations and, as a result, the read operation of the information stored on the information medium is interrupted. During stationary operation, these vibrations are so small that they can be adequately damped by means of mechanical precautions and electrical control loops. However, in the case of mobile use, such as, for example, in a car, or in the case of portable devices, the abovementioned measures no longer suffice, and use is made of a buffer which buffers the data stream for the purpose of reproduction during an interruption of the reading of the information medium. A buffer of this type is known in accordance with the data sheet of the CXD2511Q/R circuit from Sony (Vibration-Protective Memory Controller for CD Players). In order to provide a data reserve, the data are read from the CD at twice the speed and are stored in the buffer. At the same time, data are read at normal speed from the buffer for the purpose of reproduction. In order to prevent an overflow of the buffer, the reading of the information medium or the writing of data to the buffer must be continuously interrupted and the read operation must be resumed after the occupancy of the buffer has fallen below a minimum occupancy. For this purpose, the scanning or reading system must be returned to the location of the interruption and the reading of the information medium must be continuously resumed. This sequential pattern of events is continued unremittingly provided that no vibration occurs. As a result, a high number of resets or returns of the scanning or reading system are already necessary during vibration-free operation. If a vibration occurs during a reading pause, no special measures are necessary. However, if the vibration occurs during the read operation, then the filling of the buffer must be stopped immediately and the scanning system must be returned as quickly as possible to the location at which the read operation was interrupted, in order to read anew data from the information medium or the CD. Following successful conclusion of this operation, the filling of the buffer is resumed. Consequently, this jumping or resetting of the scanning system to the location of the interruption of the reading of the CD is not only necessary when vibrations occur but also necessary when vibrations do not occur in the vibration resistance mode of operation, since the read operation which is carried out at twice the speed must be interrupted when the buffer is full.
In order to write data from the information medium to the buffer following an interruption in accordance with the order present on the information medium, the reading of the CD is resumed using data prior to the last subcode address evaluated as valid by a microprocessor, and in order to start the writing to the buffer, or for the sequentially correct resumption, a comparison between the stored and the read audio data is carried out. This audio data comparison, which controls the resumption of the writing to the buffer in the event of a positive result, is carried out after the decoding and error correction and directly before the digital/analog conversion. The buffer is connected to the de-interleaving and error correction unit of the decoder and is arranged directly before the digital/analog converter.
In addition to the relatively high number of returns when no vibrations occur, a problem of this solution is the reliability of the data synchronization for starting the write operation to the buffer. The synchronization which is based on the audio data comparison is not always successful, since, owing to errors, the data bits do not always appear in the same way and accurately timed. A chronological offset also occurs when comparing very soft passages, in which only zeros are compared, or when comparing passages which contain multiple repetitions such as, for example, a sinusoidal tone at a specific frequency. The data comparison which must be carried out frequently in connection with the interruptions does not guarantee the determination of the exact synchronization instant, leads to jumps in the audio data and consequently, owing to chronological jumps in the title or disk time, does not ensure accurately timed reproduction corresponding to the stored information.
A further problem is constituted by the power consumption associated with the doubled reading speed and the frequent track jumps. The service life of batteries in portable devices is reduced approximately by half due to the decoding at twice the speed and an increased number of actuator control operations. This increased power consumption occurs even during operation without any vibrations, since the reading of the CD at twice the speed requires an interruption of the write operation and resetting or reversing of the scanning unit. Moreover, a DRAM must be provided as the buffer for the most precise data comparison possible, and it represents a considerable cost factor.
The strength to withstand shocks and vibrations acting on the device from the outside is of crucial significance for the functionality of portable CD players. In order to enjoy CDs when jogging, it is known to provide a "digital shock absorber", cf. Walter Schild: A digital shock absorber in order to enjoy CDs when jogging, in the daily newspaper Die Welt, dated 16.07.1992. In order to permit undisturbed and uninterrupted reproduction of information despite a brief interruption in the scanning of information from the CD as a result of a shock which causes the laser or scanning beam to lose the information track, the CD is driven at twice the speed and data read from the information medium are buffered. Data are then available from the buffer for the undisturbed reproduction of information during the period of an interruption. This property of portable CD players is therefore also referred to as so-called "shock-proof memory" or "shock-protection memory". The disk which is driven at twice the speed supplies data twice as fast as is necessary for the reproduction, with the result that the buffer is rapidly filled with data once more even after an interruption. In this case, however, the driving rotational speed is not constant, since CDs are read at a constant linear velocity, the so-called CLV. Since the CD has a smaller number of pits in the inner region of the disk than in the outer region, for one revolution, the driving rotational speed of the disk turntable is correspondingly reduced when reading from the inner to the outer region in order to ensure a constant reading speed. In this case, a constant reference frequency corresponding to the reading speed is used to control the rotational speed. The current speed information is taken from the frequency of the signal scanned from the information medium, which frequency forms the so-called actual value in the control loop.
If the buffer is completely full, the reading in of data is interrupted until a predetermined occupancy which warrants the further reading in of data is reached. During this time, too, the information medium is driven at twice the speed and the scanning speed is controlled by the control loop to a constant linear velocity CLV. The interruption of the read-in operation causes frequent track jumping and frequent rotational speed changes even without the occurrence of disturbances, since the scanning of the information medium following an interruption of the read operation must be resumed at a point at which the read operation was previously interrupted. The frequent track jumping and the driving of the information medium at twice the speed result in an increased power requirement, which has a particularly disadvantageous effect on the total playing time which can be attained by batteries. The total playing time which can be attained using a set of batteries in shock-proof operation is reduced approximately by half in comparison with normal operation at normal scanning speed on account of the drive at twice the speed, decoding of the scanning signal at twice the speed and an increased number of movements of the actuator or scanning system owing to increased track jumping.